TCP/IP (Transmission Control Protocol/Internet Protocol) is widely used as a connection oriented communication protocol between terminals. There is a problem in a network with large latency that throughput declines due to a congestion control function of TCP/IP.
That is, throughput of communication based on TCP/IP depends on round trip latency (Round Trip Time, hereinafter referred to as ‘RTT’) and a packet loss rate in a network between a sending side of TCP and a receiving side of TCP, and in order to improve the throughput of the TCP/IP communication, reduction of the packet loss rate is indispensable. However, in a circuit with large RTT between terminals, depending on a congestion control method of TCP/IP, it could be a primary factor by itself to deteriorate the throughput of communication.
In general, in order to perform high-speed communication via a circuit with a large delay, TCP sends a large amount of data continuously, and a sending side stores data sent in a protocol stack temporarily until an acknowledgement reply (ACK: ACKnowledge) is received from a receiving side. By this method, it is possible to improve the throughput of communication while guaranteeing integrity of the communication data. In particular in recent years, in order to improve the throughput of TCP/IP, several sending data size control algorithms with names such as TCP/Tahoe, TCP/Reno and TCP/Sack are proposed. Moreover, recently, an algorithm for long-distance high-speed transmission called High Speed TCP is proposed.
However, for a communication between terminals in the long distance, it is generally performed to communicate using a composite communication system which is composed by connecting a plurality of networks mutually. In such a case, because each network of which the communication system is composed is usually completely different in features and characteristics according to its communication distance or communication speed, the communication protocol for throughput improvement mentioned above cannot necessarily exhibit the communication performance including communication rate control between terminals. In other words, because a communication protocol has an aptitude depending on characteristics of a network, in case of a composite system of different networks, there was a problem that the throughput of the whole system could not be controlled efficiently.
An example of a technology to cope with such a problem is disclosed in Japanese Patent Application Laid-Open No. 1995-250100 (hereinafter referred to as ‘patent document 1’). In the technology described in the patent document 1, in case a plurality of LANs (Local Area Network) are connected via a wide area network, independent TCP/IP protocols are used on a LAN side and a wide area network side respectively, and both of them are converted mutually at a part of connection equipment between the LAN and the wide area network. Also, flow control of the protocol on the LAN side is terminated at the part of the connection equipment, and a parameter for flow control of the protocol on the wide area network side is set to an optimum value with regard to a transmission rate or a transmission delay of the wide area network. It is claimed that, with this technology, the throughput of the system as a whole can be held in a good state.